I'm trying to grok what's happening in that nfb loop. NFB signal hits r10 and r35 in parallel with r34, approx 20% of the signal takes the lower path, across an rc low pass (r10 and c8) at 50hz and another at about 736hz (r35 and tail capacitor c23). Meanwhile the bulk of the signal takes the high road through c15, effectively a bright cap bypassing the rc filters below it, but then gets chopped in half by the capacitive voltage divider created by c15 and c23. The remaining nfb signal is then injected into the cathodyne grid through coupling cap c20. Is any of that close to what's happening there?